Recently, studies have been conducted on merchandise management using the technology of wireless IC tags (also called RFID (Radio Frequency Identification) technology). According to this technology, a plurality of wireless IC tags can be read at the same time, so that some different identifier (a serial number or the like) in each individual product can be managed as an object, rather than assigning an ID to each box containing a plurality of individual products.
Merchandise management with wireless IC tags can manage a huge volume of attribute data and also keep track of different attribute data for different administrators for the same attribute or product. Further, most of the attribute data is publicly viewable. Thus, the attribute data may be distributed and managed on a wide-area network, so that a user can access the attribute data through the network. In order to efficiently access such attribute data on the network, it is important to minimize the amount of access. To this end, it is desirable to save attribute data once it has been accessed for reuse.
A cache system technique has been previously proposed to ensure data consistency in a system in which each of a plurality of caches holds data read from a database (see Japanese Published Patent Application No. 2000-20385). Another proposed technique to store a set of queries to a database and data obtained uses the query to simplify the management of a cache without any change in an existing application program (see Japanese Published Patent Application No. H06-274401).
However, since an ID is defined for each individual product in a wireless IC tag system, the total number of IDs becomes very large. Therefore, simply saving the attribute data once accessed is considered insufficient. For example, it is said that a store in a big supermarket chain carries three hundred thousand products with four hundred attributes (color, weight, size, date, vendor, shipping date, promotion information, advertising, image, etc.) associated with each of the products. Since a cache entry is provided on the basis of each ID of an individual product, if the number of products is three hundred thousand with two hundred individual products in stock for each kind of product, 60 million cache entries will have to be prepared for efficient access. Such a huge number of cache entries affects memory consumption when the cache is located in memory, or affects retrieval time when the cache is located on a DB. In addition, since these pieces of information are frequently replaced, the cached attribute data also needs to be frequently replaced, thereby lowering the cache hit ratio.
Japanese Published Patent Application No. 2000-20385 teaches a technique of providing a plurality of caches, but that cannot improve the cache hit ratio. On the other hand, Japanese Published Patent Application No. H06-274401 requires a determination of an inclusion relationship between a query saved in the cache and a newly received query. Since this technique deals with queries written in the syntax such as SQL, it cannot be applied to the technology of wireless IC tags that associate IDs with attribute data.